Two categories of "mental action":

1) Reason--"mind contemplating the relations borne by one thought to another."
Reason analyzes--taking things apart to determine the relations between diverse parts

2) Imagination--"mind acting upon those thoughts so as to color them with its own light, and composing from them, as from elements, other thoughts, each containing within itself the principle of its own integrity."
Imagination synthesizes--bringing together diverse elements to form a unity connecting things previously unconnected:

Poetry is "the expression of the imagination."

Poets are those people whose "faculty of approximation to the beautiful . . . exists in excess."
"Poets are the hierophants of an unapprehended inspiration . . . the unacknowledged legislators of the world."        

The poet is an author of "revolutions in opinion."
The poet "participates in the eternal, the infinite, and the one."

Poetry can have an edifying moral effect: "the great instrument of moral good is the imagination."

This moral effect comes down to love: "The great secret of morals is love; or a going out of our own nature, and an identification of ourselves with the beautiful which exists in thought, action, or person, not our own."

Imagination unites and synthesizes; poetry--"the expression of the imagination"--facilitates the process of "going out of our own nature."

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        Shelley divides "mental action" into two categories: 1) reason--"mind contemplating the relations borne by one thought to another"; and 2) imagination--"mind acting upon those thoughts so as to color them with its own light, and composing from them, as from elements, other thoughts, each containing within itself the principle of its own integrity." More simply stated, reason analyzes--taking things apart to determine the relations between diverse parts; imagination synthesizes--bringing together diverse elements to form a unity connecting things previously unconnected:
"Reason respects the differences, and imagination the similitudes of things. Reason is to the imagination as the instrument to the agent, as the body to the spirit, as the shadow to the substance."
        Shelley, having established his notion of the primacy of imagination over reason, defines poetry as "the expression of the imagination." Poets are those people whose "faculty of approximation to the beautiful . . . exists in excess." Shelley isn't speaking here merely of poets (makers) of language; rather, he widens his definition to include "authors of language and of music, of the dance, and architecture, and statuary, and painting . . . institutors of laws, and the founders of civil society." Because Shelley widens his definition of poetry (making) to "the expression of the imagination," he ultimately extends the label "poet" to all of us. "Making" and imagining are what we all do, in one way or another. Shelley's democratic instincts here are the reverse of the hierarchical ideas of Plato.
        The poet--in this case the "great" poet--is an author of "revolutions in opinion." The poet "participates in the eternal, the infinite, and the one"--revealing the Platonic/Plotinian roots of Shelley's thought. Poetry "awakens and enlarges the mind itself by rendering it the receptacle of a thousand unapprehended combinations of thought." Poetry can--and does--have an edifying moral effect, but this effect is not to be achieved through simple didacticism; rather, "the great instrument of moral good is the imagination." Imagination unites and synthesizes, while reason divides and analyzes. Shelley's morality finally comes down to love: "The great secret of morals is love; or a going out of our own nature, and an identification of ourselves with the beautiful which exists in thought, action, or person, not our own." This is precisely the effect poetry achieves--an ecstasy (literally a standing outside the self), a transcendence of the Self through identification with that which is Other. This is a personalized, humanized, less exclusively subject/object oriented adaptation of Schopenhauer's notion of aesthetic arrest. The identification can be with not merely an object, such as a tree, or a painting, or even a poem, but with another person:
"A man . . . must put himself in the place of another and of many others; the pains and pleasures of his species must become his own."
This identification of Self with Other, this "love," is the moral effect--if not the moral purpose--of poetry.
        Shelley credits the poetry of the "Provencal trouveurs," Dante, Petrarch, and Shakespeare with raising the status of women--a claim which may be somewhat overstated.
        Shelley goes on to defend poetry and poets against the challenges of "reasoners and mechanists" who allege that reason is more useful than imagination. Pleasure is what "a sensitive and intelligent being seeks." He divides pleasure into two categories: 1) the "durable, universal and permanent"--"whatever strengthens and purifies the affections, enlarges the imagination, and adds spirit to sense"; and 2) the "transitory and particular"--"that which banishes the importunity of the wants of our animal nature," surrounds men with the "security of life," and generally proceeds from "motives of personal advantage." Shelley identifies the former pleasure with imagination, while identifying the latter pleasure with reason.
        Shelley takes the Platonic view of poetry as primarily a product of inspiration, and he assigns the poet a divine function:
"Poets are the hierophants of an unapprehended inspiration . . . the unacknowledged legislators of the world."